for y=0,height-1,1 do
  for x=0,(width/2)-1,1 do
    l0,a0,b0=get_lab(x,y)
    l1,a1,b1=get_lab((width/2)+x,y)
    set_lab(x,y,l0,a1,b1)
    set_lab((width/2)+x,y,l1,a0,b0)
    end
    progress(y/height)
  end

